In the field of semiconductor manufacturing, it is well known to use a low energy, low dose ion implantation process to incorporate impurity atoms into a semiconductor substrate. While conventional ion implantation is capable of providing implant regions in most devices, it does not always provide sufficient ion placement, resolution and shallow depths that are typically required even for today's deep-submicron semiconductor devices; and will not be adequate for more aggressively scaled-down devices in the near future.
For example, in the context of silicon VLSI technology, the volume bounded by a sub-0.1 .mu.m channel length, L, metal oxide semiconductor field effect transistor (MOSFET) having sub-micron gate widths, W, where W is less than 0.10 .mu.m, and which is doped with at most 1.0.times.10.sup.18 atoms/cm.sup.3 (implying a maximum depletion depth of 0.10 .mu.m) will contain on the order of 25 to 100 dopant atoms. The percent control, C, of the integrated dose of this threshold implant is roughly proportional to [1-sqt(N)/N], wherein N is the number of dopant atoms in the depletion region. Thus, when N=1000, the percent control, C, is roughly 99%; when N=100, C is equal to 90%; and when N=25, C is about 80%. Clearly, the degree of dose control drops precipitously for very small semiconductor devices.
In view of the current trend towards smaller and smaller semiconductor devices, there is a need for developing new and improved methods for incorporating dopant atoms into a semiconductor substrate. Such methods should provide improved controllability as well as resolution while limiting the dopant drive-in depth to within 1 to 3 monolayers from the substrate's surface.
One known alternative to using conventional low energy, low dose ion implantation is to employ a Scanning Tunneling Microscope (STM). In this prior art technique, a voltage is applied between the tip of the microscope and the semiconductor sample. When the tip of the Scanning Tunneling Microscope is brought in close proximity to the semiconductor sample (i.e. gap .ltoreq.1 nm), ionized atoms accelerate through the gap due to the electric field in the gap and are implanted into the semiconductor sample. While Scanning Tunneling Microscopy might be used in some applications, it may not afford the controllability in dopant placement due to electric field lateral dispersion and dopant drive-in depth required for today's generation of sub-micron semiconductor devices. A bigger problem is the need for high vacuum in which to operate the STM.